It is known that most high-speed digital-transmission systems (e.g. 144 kbit/s systems), designed to operate with subscriber's lines in the national or private networks, require an equalizer for the received signal.
In fact, the transmission line introduces attenuations of various values depending on the frequency of the transmitted signal, thereby distorting the transmission.
In addition the receiver must afford the possibility of adjusting the gain, since the transmission lines may have different lengths dependent on the subscriber's distance from the exchange, and hence the introduced attenuations are different and cannot be predetermined.
A continuous or step manual adjustment requires the intervention of an operator upon installation, and, owing to possible variations in the line characteristics with time, during the operation. The high cost of such interventions and the necessity of maintaining the best system performances over time render the use of automatic gain controls particularly advantageous for an automatic receiver adaptation to the amplitude of the received data signal.
In order to automatically carry out such automatic operations the amplitude has to be precisely estimated.
According to known techniques, such estimates are made by measuring the dc voltage obtained from the signal which is subjected to a non-linear operation, usually a detection of the envelope obtained by rectifying, possibly followed by a low-pass filtering operation. However, these operations have a number of disadvantages, due both to low sensitivity of non-linear components in handling very weak signals, and to the dispersion of the characteristics of the components themselves. In fact, non-linear components with strictly equal parameters, i.e. such as to ensure a good reproducibility in the accuracy and sensitivity properties of the detector, cannot be achieved by present integrating techniques.
There is also the difficulty of reproducing a precise reference voltage with which the voltage to be measured may be compared.